Conventionally, a combustion apparatus, such as an internal combustion engine, uses a spark plug for igniting an air-fuel mixture through spark discharge. In recent years, in order to meet demand for high output and low fuel consumption of a combustion apparatus, a plasma jet ignition plug has been proposed, since the plasma jet ignition plug provides quick propagation of combustion and can more reliably ignite even a lean air-fuel mixture having a higher ignition-limit air-fuel ratio.
Generally, the plasma jet ignition plug includes a tubular insulator having an axial bore, a center electrode inserted into the axial bore in such a manner that a forward end surface thereof is retracted from a forward end surface of the insulator, a metallic shell disposed externally of the outer circumference of the insulator, and an annular ground electrode joined to a forward end portion of the metallic shell. Also, the plasma jet ignition plug has a space (cavity) defined by the forward end surface of the center electrode and an inner circumferential surface of the axial bore, and the cavity communicates with an ambient atmosphere via a through hole formed in the ground electrode.
Such a plasma jet ignition plug ignites an air-fuel mixture as follows. First, voltage is applied between the center electrode and the ground electrode, thereby generating spark discharge therebetween and thus causing dielectric breakdown therebetween. In this condition, high-energy current is applied between the center electrode and the ground electrode for effecting transition of a discharge state, thereby generating plasma within the cavity. The generated plasma is discharged through an opening of the cavity, thereby igniting the air-fuel mixture.
Meanwhile, according to a conceivable technique for implementing further superior ignition performance, higher energy is imparted to current to be applied after spark discharge, for generating larger plasma. However, when high-energy current is applied, the center electrode is apt to be eroded, potentially resulting in a rapid increase in voltage required for spark discharge (discharge voltage).
In order to cope with the above problem, there is proposed a technique for implementing excellent ignition performance even with relatively-low-energy current through provision of a throttle in the cavity by means of provision, on the inner circumferential surface of the cavity, of a stepped portion or a diameter-reducing portion whose diameter reduces along the forward direction (for example, refer to WO2008/156035A1 “Patent Document 1”).